$3np + 6nq - 10n - 10 = p - 7$ Solve for $n$.
Combine constant terms on the right. $3np + 6nq - 10n - {10} = p - {7}$ $3np + 6nq - 10n = p + {3}$ Notice that all the terms on the left-hand side of the equation have $n$ in them. $3{n}p + 6{n}q - 10{n} = p + 3$ Factor out the $n$ ${n} \cdot \left( 3p + 6q - 10 \right) = p + 3$ Isolate the $n$ $n \cdot \left( {3p + 6q - 10} \right) = p + 3$ $n = \dfrac{ p + 3 }{ {3p + 6q - 10} }$